This invention relates to hydraulic systems of the type including one or more flow and pressure compensated pumps for providing hydraulic fluid under pressure to parallel hydraulic circuits, each including a work performing means.
Increasingly, hydraulic systems having work performing devices subjected to variable loading are utilizing one or more flow and pressure compensated pumps for the reason that such pumps maximize system efficiency within their capacity and yet provide a considerable reduction in energy requirements. In the operation of many such systems, only infrequently are the various work performing devices simultaneously subjected to maximum load conditions with the result that it is uneconomical to provide pump capacity that is sufficiently high that the fluid requrements of all work performing devices can be fully met when all are simultaneously being subjected to maximum load conditions. As a consequence, most systems of this type are provided with a pump capacity that is less than the theoretical maximum required for the specific situation wherein all components are subject to maximum load.
Nonetheless, during the operation of such systems, this infrequently occurring happening will take place periodically. And, where one of the work performing devices provides a relatively low resistance to the passage of hydraulic fluid as opposed to one or more of the other work performing devices, the demand for fluid by the low resistance work performing device will be substantially fully satisfied with the consequence that flow to the higher resistance work performing devices will be severely cut back thereby severely diminishing the ability of the high resistance work performing device to perform its intended function during the occurrence.
All too frequently, the nature of the system will be such that, in the usual operation, the function produced by the low resistance work performing device is one of lesser importance to the operation being performed by the system while the function being performed by the high resistance work performing device is of considerably greater consequence and importance.
As a result, quite undesirably, a most desirable function cannot be satisfactorily performed while a function of less importance can be.
In order to solve the difficulty, the prior art has suggested the provision of priority devices whereby the demand of the work performing devices performing the more important functions is first satisfied, and only after such satisfaction has been attained, is the demand of the work performing devices performing functions of lesser importance attended to. Typically, the priority determination is accomplished by means contained within flow control valves which directly interconnect the pump or pumps and the work performing means. As a consequence, the priority determining means are necessarily large since they are interposed in hydraulic circuits having large flow rates and must be designed to withstand the high pressures typically associated with many hydraulic systems.
In addition, many such hydraulic systems employ plural pumps, each for normally providing fluid under pressure to an associated group of work performing devices. In order to maximize efficiency, means have been provided whereby the output of one pump may be transmitted to work performing devices not normally associated therewith so as to maximize the use of the total pumping capacity employed in the system.
Heretofore, such transfer means have utilized spool valves which tend to be rather expensive and which tend to be relatively leaky as compared to other types of known valves.